Heater for a diffusion pump



July 5, 1960 e. H. BANCROFT ETAL 2,943,783

HEATER FOR A DIFFUSION PUMP Filed April 1. 1957 I I J i I g 60 caoleaally l orr ORDON F. GEROW BY ONALD L. SEVENSON ATTORNEYS July 5, 1960 e.H. BANCROFT A 2,943,783

HEATER'FOR A DIFFUSION PUMP Filed April 1, 1957 2 Sheets-Sheet 2INVENTOR.

NSO/V AT TOR/VEYS GEORGE H. BANCROFT coma/v P smow BY DONALD L. STEVE 02 w 2 |I M :..l\ n w I i w IHWH m .5 i I! I,

FIG. 3.

HEATER, FOR A, DIFFUSION PUMP George H. Bancroft-and Gordon P. Gerow,Rochester, and'Donald L. Stevenson, Albion, N.Y., assignors, by mesneassignments, to. Consolidated Vacuum Corpora tion, Rochester, N. Y., acorporation of New York.

' Filed Apr, 1,1957, Ser.No. 649 ,89.0 2 Claims. 121.230.5401

This invention relatesto diffusion typevacuum pumps, and moreparticularly, is concernedwith improved heater apparatus for such pumps.T

Diffusion pumps for operation at highvacuums are well known. Suchpumpsoperate on the principle that a liquid having relatively heavymolecules is vaporized by raising its temperature. The vaporwithits-heavy molecules is directed by suitable jets indirection away fromthe region to be evacuated towards a mechanical forepump. Theaccelerated molecules of vapor compressthe gas molecules ahead of thejet, forcing them toward the mechanical forepump and thereby reducingthe pressure within the evacuated region. The vapors are recondensed onthe cooled walls of the pumpwhere the liquid is permitted toreturn tothe bottom of the pump to be reheated and vaporized.

It has been the general practice heretofore toprovide a reservoir ofoil, for example, at: the bottom of the diffusion pump and apply heat tothe bottom of the diffusion pump by means of a flat electric heaterpositioned under the bottom of the pump, the heat. being transferred. tothe oil by conductionthrough the walls of the pump. The oil is thuscaused to boil, producing the vapors which rise in the pump and aredirected through orifices which provide the proper jets of the heavymolecule vapors of the oil. In addition to being an ineflicient means oftransferring heat to the pump liquid/such known heating methods do notcause the liquid to boil uniformly. Local hot spots exist which producegreater boiling activity in certain portions of the liquid; The liquid:does not release the vapors in a: steady fashion. Greater agitation ofthe liquid in spots causes liquid droplets to be carried along with thevapor. The result is erratic and inefficient operation of the pump.

The present invention. provides improved heating means for a diffusionpump which applies heat more uniformly through the volume of the liquid.Moreover, the heater of the present invention provides a means for heating the vapors after they leave the surface of the liquid. In thismanner the vapors are super-heated to increase the energy of the vapormolecules, which in turn increases the velocity of the molecules in thejets of the pump to increase the pumping efiiciency.

To this end, the present invention provides novel heat ing means in adiifusion pump having a cylindrical outer wall and a bottom wall, and ajet element having a lower cylindrical wall portion positionedconcentrically within the outer wall of the pump against the lower endthereof. The heater means for boiling the diffusing liquid of the pumpcomprises a lower cylindrical casing which projects upwardly from thebottom wall of the pump within the hollow jet element. The casing isjoined to the bottom wall in a hermetically sealed joint, the projectingupper end of the casing being closed off and hermetically sealed. Acartridge-type electric heating element is positioned in the casing, anda plurality of heat-conductive fins joined to the casing, the finsprojecting radially outward from the casing. Part of the casing andprojecting fins is below Patented Jul-y. 5,; 1.9 9:

the. surface of the liquid and part is above the surface; of the liquid,whereby the vapors from the boiling liquid; are superheated by thecasing and fins, as they rise, in,. th e, jet element.

For a better understanding of the invention, reference should be had tothe accompanying drawings wherein: i

Fig. 1 is an elevational view. of .a' diffusion pumputiliz: ingtheimproved heater of the present invention;

Fig. 2 is a sectional view of the'dilfusion pump. showing the interior.construction thereof;

Fig. 3Jis a sectional view takensubstantially; onthe line..3-3 of Fig.2; and

Fig. 4..is a sectional view similarlto Fig. 3. but

a. modified construction. of.:the heating means.

Referring to the drawings in detail, the numeral: it) indicatesgenerally. thehousing of the pump, which includes a hollow cylindricalportion 12 preferablyjm'ade of metal, and a lower end wall 14. Theup'per endtwall of the housing. 10 includes a'metal' cap 16. which fitsintothe endof the cylindricalwall 12. A seal is made by means ofanO-ring 18 which is squeezed outwardly against the inner Wall of thecylindrical portion 12 of the housing 10 by means of an annular plate 20having a threaded stud 22- secured thereto. A nut24 engages the stud 22.When tightened up, the nut causes the O-ring to be squeezed out betweenthe cap 16, and the annular plate 2th in sealing relationship with thecylindrical wall portion 12. V

The discharge side ofthe pump is provided by a pipe 26, oneend of'whichopens into the lowerportion offth'e housing 10. A flanged coupling 28provides, means for coupling the pumpto the system to be evacuated- TH:flange coupling 28 has an opening therethrough communieating with theupper end of the vacuum pump housing. A cooling coil in the form of acontinuous metal tube 30 is Wound around the exterior of the pumphousing and the discharge pipe 36. A suitable coolant canbe-circulatedthrc'iughthetube 30 to provide means for cooling the wallsof the pump. I

The interior of the pump includes a jet element indie cated generally at32. The jet element is preferably constru-cted of three cylindricalsections 34, 36, and 38 which are successively of diminishing diameter.The three cylindrical sections are located along a common axis withtheir ends being joined in. concentric overlapping relationship. Thusthe cylindrical members 34 and 36 are joined by relatively thin spacers40 and the cylindrical sections 36 and 38 are similarly joined byspacers 42.

The heater assembly to which the present invention is particularlydirected is located in the lower end of the housing 10. The heaterassembly includes a hollow cylindrical casing 44- which projectsupwardly into the housing from the lower end wall 14. The lower end ofthe casing 44 is joined around its outer periphery to the bottom plate:14, a hole being provided in the bottom plate 14 to receive the casing44. By suitable welding or brazing of the joint between the bottom plate14 and the casing 44, an hermetically sealed joint is providedtherebetween. The upper end of the hollow casing 44 is closed off by anend plate 46. Thus the hollow interior of the casing 44 is open toatmospheric pressure, while the outer wall of the casing 44 forms partof the wall of the evacuated pump interior.

A plurality of radially projecting conductive fins, such p 3 the liquidcharge, preferably more than half the length of the fins and casingbeing above the liquid level. The liquid level is indicated by thedotted line 50.

Heat is supplied to the finned heating structure by means of acartridge-type heater 52. Such type heaters are well known and may beobtained in a great range of sizes. The cartridge heater includesyanouter metallic wall 54 which has an outer diameter substantially equalto butjslightly less than the inner diameter of the casing 44, wherebythe cartridge heater 52 can be inserted into the interior of the casing44. A pair of leads 56 extend out of the end of the cartridge heater 52for connecting toa suitable source of electric power. The cartridgeheater 52 is held in position by means of a collar 58 secured tofthe'bottom plate 14. The collar 58 extends around the opening through whichthe cartridge heater 52 is inserted int o the casing 44; A set screw 60in the collar 58, when tightened, locks the cartridge heater intoposition.

- 'In the modification of Fig. 4, the fins 48 are provided at theirlower ends with portions 49 extending substantially parallel to thebottom wall. This increases the heating area of the fins and alsoprovides increased thermal agitation and movement of the liquid for moreuniform of the pump with a substantial portion of the surface areageneration of vapor over the whole surface of liquid. 7

' It will be noted that the finned heating structure is closelysurrounded by the wall of the cylindrical section 34. This wall is ofcourse cooler than the heater and tends to remove heat from the liquidcharge. fining and cooling theliquid in the immediate vicinity of theheating structure, strong convection currents are produced in the liquiddue to the combined heating of the heater and the cooling of thesurrounding cylindrical wall.

This decreases the time anyof the liquid is exposed to the heatingsurface. .The result is more uniform heat distribution through theliquid charge, and the elimination of localized hot spots which causeisolated areas element positioned in the casing, a cylindrical wallencircling and arranged concentrically with said casing, a plurality ofheat-conductive fins joined in heat conductive relationship to thecasing, and extending lengthwise of the casing, the fins projectingradially outwardly from the casing within and extending substantially tothe inner surface of said cylindrical wall to form a compartmentedregion for the liquid, the casing and fins projecting above the bottomwall of the pump and extending vertically partly below. and partly abovethe level of the liquid charge in the pump, whereby the vapors from theboiling liquid in said compartmented region are superheated by By conofturbulence otherwise present in conventional heating arrangements.

We claim: 1. A diffusion pump having a cylindrical housing and a bottomwall, and a jet element positioned vertically and concentrically withinthe housing wall of the pump comprising heater means for boiling thedilfusion liquid of the casing and fins as they rise within the jetelement.

2. In a diffusion pump having an outer wall and a bottom wall, and a jetelement having a lower wall portion positioned within the outer wallofthe pump, means for boiling the diflfusion liquid of the pump whichnormally collects in the region of the bottom wall, comprising anelectric heater projecting up from the bottom wall of the pump, theheater being secured to and supported by the bottom wall in ahermetically sealed joint, the heater being formed with a heatingsurface extending above and below the surface of the liquid in thebottom of the heater being above the liquid surface, a plurality of finsextending lengthwise of the heater and projecting radially therefrom,and means defining a confining wall closely surrounding the heater andassociated fins, each of said fins including a lower portion extendingat an angle to the remainder of the fin.

References Cited in the file of this patent UNITED STATES PATENTS1,471,012 Snook Oct. 16, 1923 2,121,599 Kilgore et al. June 21, 19382,246,327 Slepian June 17, 1941 2,305,947 Armstrong et a1. Dec. 22, 19422,432,169 Morgan et -al. Dec. 9, 1947 2,437,897 Stoltenberg et al Mar.16-, 1948 2,505,953 Flosdorf May 2, 1950 2,508,765 Morand May 23, 1950 r2,630,266 Lawrance Mar. 3, 1953 2,721,025 Lawrance'et a1. Oct. 18, 19552,785,274 Andrus Mar. 12, 1957 FOREIGN PATENTS 100,429 Austria July 10,1925 338,971 Germany July 9, 1921 686,659 Great Britain Jan. 28, 1953

